Pressure-Driven Energy Band Gap Narrowing of λ-N₂

Probing the energy band gap of solid nitrogen at high pressures is of importance for understanding pressure-driven changes in electronic structures and insulator-to-metal transitions under high pressure. The λ-N₂ formed by cold compression is known to be the most stable one in all solid nitrogen phases observed so far. By optimizing the optical system, we successfully measured the high-pressure absorption spectra of λ-N₂ covering the polymeric-nitrogen synthetic pressures (124 GPa–165 GPa). The measured optical band gap decreases with increasing pressure, from 2.23 eV at 124 GPa to 1.55 eV at 165 GPa, with a negative pressure coefficient of −18.4 meV/GPa, which is consistent with the result from our ab initio total-energy calculations (−22.6 meV/GPa). The extrapolative metallization pressure for the λ-N₂ is around 288(18) GPa, which is close to the metallization pressure (280 GPa) for the η-N₂ expected by previous absorption edge and direct electrical measurements. Our results provide a direct spectroscopic evidence for the pressure-driven band gap narrowing of solid nitrogen.